The Study Of Modified G-C₃N₄ For Photocatalytic Nitrogen Fixation

The photocatalytic N₂ reduction reaction（NRR）to replace the high energy-consumption Haber-Bosch reaction is the future development trend of industrial ammonia synthesis.However,the low apparent quantum efficiency and catalytic stability of current photocatalytic NRR make it impossible to be applied to industrial production.In order to design an efficient NRR photocatalyst,building a large number of active centers to weaken the stable N≡N triple bond is a key consideration.In addition,inhibiting the competing H₂ precipitation reaction（HER）to improve the selectivity of the reaction is also very important to improve the efficiency of the NRR reaction.Graphite carbon nitride（g-C₃N₄）is considered a promising photocatalyst.However,the short lifetime of photogenerated carriers and the lack of reactive centers are the main reasons for the low photocatalytic efficiency of NRR.Defect engineering creates crystal defects on the surface of the material.The defect site is a common catalytic reaction center and causes changes in the properties of the material itself.In addition,interface engineering uses the photocatalyst and other NRR active materials to construct a heterogeneous structure to promote rapid charge transfer to inhibit the recombination of electrons and holes.At the same time,it may also introduce reaction sites that are more active for NRR,which is also considered to be NRR light.An efficient and simple modification method for catalysts.In this paper,the defect-rich g-C₃N₄ was prepared by the citric acid-assisted thermal polymerization method,which achieves an effective exfoliation of the carbon nitride stacked block to the single-layer porous nanosheet.Through characterization methods such as FT-IR,XPS and EPR,it is determined that the nitrogen defect exists in the sp²N of the carbon nitride ring.In order to study the influence of nitrogen defects on the nitrogen fixation performance,the photoelectric properties and nitrogen adsorption capacity of defect-rich g-C₃N₄ were characterized.It was found that the generation of nitrogen defects increased the electron transport rate of carbon nitride and acted as a trap for photogenerated electrons to achieve effective separation of electrons and holes;in addition,nitrogen defects also increased the adsorption capacity for nitrogen molecules,which serve as a possible catalytic N₂ reduction reaction site.Ti₃C₂ MXene is used as the active center to further modify defect-rich carbon nitride through interface engineering.Taking nitrogen defects introduced by citric acid modification as anchoring sites,MXene fills the nitrogen defects with oxygen-containing terminals to constructe a two-dimensional heterostructure by C-O-Ti bonding after gentle heat treatment.Research on the photoelectric properties of the composite catalyst found that the electric field established at the heterogeneous interface greatly reduces the interface charge transfer resistance,thereby inducing photo-generated electrons to migrate to the high-conductivity active Ti sites of MXene.By designing photonitrogen fixation performance experiments that expose different sites,it is found that the edge Ti atoms exposed by MXene are the active sites of NRR;and the light hydrogen production competition experiment and photocurrent test are used to prove that the edge Ti sites have priority to nitrogen molecules Activated to show excellent NRR selectivity.The redox cycle related mechanism of multi-valence Ti species is proposed to reveal the evolution of Ti species during the adsorption,activation and dissociation of nitrogen molecules on active Ti sites:Under the irradiation,the electrons on the valance band of CN get enough energy to leap into the conduction band.Due to the construction of the CT-CN heterostructure,the photoinduced electrons rapidly shuttle through the interface and transfer to the active Ti sites in the edge planes of Ti₃C₂.Then the high-valance Ti accepts electrons to turn into the low-valance active Ti which possess the filled 4s orbital and empty 3d orbital leading to a strongly N₂ chemisorption.The electron-rich low-valance Ti donate electrons to N₂ adsorbates lead to the N≡N was polarized.Finally,the weakened N≡N can simply couple with H released by slitting water on the terminal oxygen sites of Ti₃C₂The Fe-modified MXene derivative is used as the active center to continue to further modify the defect-rich carbon nitride.The composite catalyst presents a multiple heterogeneous structure:on carbon nitride,Ti₃C₂ coated with a carbon layer is supported on carbon nitride to grow nano-sized anatase and rutile mixed crystal Ti O₂ by in-situ oxidation,and it is dotted with highly uniformly dispersed Fe single metal atoms.Through the research on the photoelectric performance of the composite catalyst,it is found that this multi-element heterojunction makes the two-phase photo-responsive material carbon-coated mixed crystal Ti O₂ and CN form a photoresponse synergy effect,realizing the photoresponse of the composite photocatalyst Together with the enhancement of the reducibility of photogenerated electrons,it provides a powerful driving force for the NRR reaction.Using different active sites for nitrogen fixation performance experiments,it is proved that Fe-N₄ single metal atomic sites are used as active sites to achieve effective adsorption and activation of nitrogen molecules,and provide efficient reaction sites for NRR reaction.